// 实现every
Array.prototype.every = function(fn, value){
    if (typeof fn !== "function") {
        return false;
    }
    var arr = this;
    for (var i = 0; i < arr.length; i++) {
        var result = fn.call(value, arr[i], i, arr);
        if (!result) return false;
    }
    return true;
}